KiLROE — The Shannon: its Course and Geological Histor?/. 89 



2nd. That tlie all but latest stage of erosion in the gorge was 

 effected by glacial action, which alone could have caused the abnormal 

 deepening of the bed, as at present. 



3rd. That upon the melting of the glacier which filled the gap, 

 the river was slightly deflected from its original course by moraine 

 matter at Killaloe ; and, south of Birdhill, it encountered the moraines 

 formed during earlier melting of the glacier, which diverted its flow 

 opposite O'Briensbridge into its present channel. Had the moraines 

 not existed, the river would probably have flowed directly southward 

 and entered the flat tract now filled with peat and alluvium, where it 

 would have been joined directly and normally by the N'ewport and 

 Annagh rivers. 



First, with regard to the comparative heights of the valley of 

 probable discharge by Scarriff at the lowest point, and of the bed at 

 Killaloe, the water-level in Loughanillon near the watershed, and of 

 the sluggish Cloghan rivcr^ which drains it, is 133 feet. The water- 

 shed line passes over a low drift parting between that lakelet and 

 Lough Bridget, which has a surface-level of 115 feet. The rock-floor 

 beneath the ridge, and forming the river and lake bottoms, would 

 no doubt be considerably nearer to the 100-feet level, that of the 

 present natural point of escape at Killaloe ; and, as we shall have to 

 take account of great ice-erosion in the gorge, which must have con- 

 siderably lowered this point of discharge below the level at which it 

 probably stood in pre-glacial times, there can be little question that 

 the Scarrifi valley formed the presumed alternative course for Lough 

 Derg water prior to that erosion. In the section referred to on 

 page 92, the heavy dash-dot line might possibly be supposed to be 

 the bottom of a U valley formed by a certain late stage of giaciation 

 -from 160 to 230 feet above present datum. The rates of lowering 

 of valleys by giaciation have been estimated at 2 cm. to 3 cm. per 

 ear in the Alps, and the time taken for the formation of some of the 

 principal valleys has been calculated at 50,000 to 70, 000^. years, or 

 2,500 years for the latest stage of giaciation. ^ If we take half the 

 lesser rate, and suppose the erosion to have continued 12,500 years, 

 the river bottom at Killaloe might have been lowered even more than 

 to its present level in this time. 



^ With a fall of four feet in three miles, towards Scarriff, emptying hito 

 Lough Derg. The water south-west of Loughanillon flows towards^'the Shannon 

 estuary. 



^ Die Gletscher, by Dr. Hans Hess, pp. 187, 376-7. 



U.I. A. PROC, VOL. XXVI., SEC. B.] M 



